Imidazoles and simple imidazole derivatives are being used more and more as hardening agents for epoxy resins. They provide long pot life, high heat distortion temperatures, economical performance based on low PHR requirements and lower toxicity than amines. They are useful accelerators (catalysts) for anhydride cure and bisphenol-A cure of epoxy resins. The imidazole ring can be utilized in numerous reactions and these derivatives can be used for specialty epoxies. For example, delayed action can be obtained by acylating imidazoles with polychlorinated benzoyl chloride. See U.S. Pat. No. 4,436,892. ##STR1##
Early synthesis of imidazoles involved the reaction of 1,2-dicarbonyl compounds with ammonia and aldehydes to produce low yields according to the equation: ##STR2##
The yields were increased when the reaction was carried out in organic acids with ammonium acetate. Radziszewski, R. Ber, 15, 2706 (1882).
Imidazole oxalate, fumarate, adipate and phthalate and 4-methylimidazole, 4,5-dimethylimidazole and 2-isopropylimidazole have been made from the .alpha.,.beta.-dicarbonyl compounds, as demonstrated in U.S. Pat. No. 3,715,365.
Werdenhazen, R., and Rienacker, H., Ber. 72, 57 (1939) demonstrated the production of imidazoles from .alpha.-hydroxyketones under the influence of ammoniacal cupric acetate and aldehydes represented by the following: ##STR3##
In "New Methods of Preparative Organic Chemistry", Vol. 3, p. 241, Academic Press. N.Y., 1964, H. Budereck, et al. describe another method involving formamide synthesis according to the equation: ##STR4##
Some 2-methylimidazoles have been prepared using acetamides; however, yields were reduced when amides other than formamide were used.
One of the more attractive methods for producing imidazoles is discussed in U.S. Pat. No. 2,891,966. This involved the reaction of a 1,2-diamine with carboxylic acids. For instance, ethylene diamine was admixed with a slight excess of acetic acid, permitting an exothermic rise which dissolved the reagents. The homogeneous solution was cooled to about 100.degree. C., then charged through a continuous reactor equipped with a preheater section, a reactor section filled with platinum. Then the reactor was heated to 430.degree. C. and hydrogen added. The vaporous reaction effluent was condensed to obtain crystalline 2-methyl imidazole.
A French patent describes the purification of 2-methyl imidazole by codistilling it with 1- or 2-methylnaphthalene and then washing with pentane or toluene. See French P. No. 1,362,689 (1964).
In other work, the diamine is converted to 2-alkyl imidazoline, and then dehydrogenated to the corresponding imidazole compound by dehydrogenation over a nickel catalyst. For example see U.S. Pat. Nos. 2,399,601 and 2,404,299 on the preparation of imidazoles by heating imidazolines with Raney nickel catalysts. The reactions were carried out at 225.degree.-235.degree. C. The yields were not reported in some cases or varied widely.
H. A. Green, of Air Products has demonstrated that 1,2-diamines can be reacted with aldehydes and then heated over a platinum-alumina catalyst at 370.degree. C. to give imidazoles. In the case of ethylenediamine and propionaldehyde a 56% yield of 2-ethyl imidazole was obtained. See U.S. Pat. No. 3,037,028, May 29, 1962.
In U.S. Pat. No. 3,037,028, using another vapor phase reaction, Green demonstrated that imidazole could be obtained from ethylenediamine and formamide using a large volume of hydrogen. See also U.S. Pat. No. 3,255,200. The catalyst used was platinum-on-alumina and alumina or cobalt molybdate were shown to be ineffective. Treating ethylenediamine with methyl formate at 25.degree.-30.degree. C. gave 98.5% diformyl derivate.
In Ger. Offen. DE No. 3,009,605, diformyl derivate was passed with nitrogen over 6:14 NiO:MoO.sub.3 at 400.degree. C. to give 65.7% imidazole with 99.3% conversion. The yield remained constant after 250 hours use of the catalyst.
Imidazolines can be dehydrogenated to imidazoles at 250.degree.-500.degree. C. over MoO.sub.3 and NiO and/or CoO and Al.sub.2 O.sub.3, SiO.sub.2 and/or alkaline silicate catalysts. See DE No. 3,009,631. This reaction has been used to make a variety of 2-alkylimidazoles substituted with long chain fatty acids. As noted, the minimum temperature requirement is 250.degree. C.
Another route to imidazoles involved the reaction of a nitrile with a diamine over a copper salt to give imidazolines which were then dehydrogenated with an aluminum-zinc oxide catalyst to give imidazoles. See DE No. 3,236,598-A to BASF.
In U.S. Pat. No. 4,409,389, hexamethylene-tetramine was reacted with formamide at 140.degree. C. to give a bis-formamide. The bis-formamide with dicarbonyl compounds and 2 moles of mineral acid yields imidazole acid salts. ##STR5##
In Ger. Offen. No. 1,952,991, imidazole was made in 53% yield by passing a solution of ethylenediamine and formic acid over a Cd-Cu chromite catalyst at 480.degree. C. This method required quite high temperatures and the yield was very moderate.
Imidazolines have been dehydrogenated with sulfur, manganese dioxide, etc.
It is noted that most methods for producing imidazoles which are found in the art require higher temperatures and the yields reported are nowhere near 99%. Further one does not observe the higher yields and mild temperatures in the same work. For instance, no previously used process for producing imidazoles allows for as great as 99% yield or higher with almost no by-products using temperatures of around 200.degree. C. or less.
It would be a substantial advance in the art if imidazoles could be prepared in essentially quantitative yields using mild conditions. Such a process would be particularly attractive economically if essentially no by-products were formed.